The invention is based on a priority application EP 02 360 328.5 which is hereby incorporated by reference.
The present invention relates to a receiver for an optical time-division multiplexed (OTDM) pulse train in which the pulses have alternating polarizations. The invention further relates to a method for receiving such an OTDM pulse train.
In typical OTDM systems, an optical pulse source on the transmitter side generates a pulse train with a channel bit rate BC that equals the base rate of electronic data streams fed to the OTDM system. This optical pulse train is coupled into n optical branches in which modulators are driven by the electrical data streams. Each modulator imprints the incoming data stream on the pulse train, thereby generating an optical data signal with the channel bit rate BC. The n optical data signals, which represent n different channels, are interleaved by a delay-line multiplexer on a bit-by-bit basis (bit interleaving TDM) or on a packet-by-packet basis (packet interleaving TDM). The multiplexer generates a multiplexed optical data signal with a multiplex bit rate BM=n×BC. The multiplexed signal is then launched into a transmission medium, for example a single-mode optical fiber.
On a receiver side, an optical demultiplexer usually de-interleaves the channels, because electronic devices are not capable of directly processing signals with bit rates BM. The demultiplexed signals with the channel bit rate BC are finally reconverted by opto-electronic devices into electric signals for further processing.
In ultra-high-speed OTDM transmission systems having bit rates of more than 40 Gbit/s, pulse durations are extremely short. For a 160-Gbit/s system, for example, the time slot for a single bit is only 6.25 ps wide. As OTDM transmission systems have to use return to zero (RZ) pulses, i.e. pulses that return to zero power level within each time slot, the width of a pulse in ultra-high-speed OTDM transmission systems is even shorter, namely about one half of the time slot width.
Such extremely short pulse durations pose very high demands on the demultiplexers that are one of the key components of an OTDM receiver. Demultiplexers that are capable of separating pulses in ultra-high bit rate optical transmission systems require very short switching windows and a high extinction ratio.
One approach that has been proposed to facilitate the separation of pulses in OTDM pulse trains is to combine optical time-division with polarization-division multiplexing. Optical polarization-division multiplexing (PDM) is a type of optical multiplexing that multiplexes several polarized optical pulse trains having different polarizations into single optical pulse train. Usually two pulse trains with orthogonal polarizations are bit interleaved such that the polarizations of the pulses of the resulting pulse train alternate.
Standard single-mode optical fibers support PDM because two orthogonal states of polarization can exist in the fundamental mode of such fibers. The relative orthogonal nature of the polarization is preserved in the fibers although the state of polarization (SOP) of the optical pulse trains is randomized as the pulse train propagates through the fiber. This assumes that polarization effects, such as polarization mode dispersion (PMD) and polarization-dependent loss (PDL), are not significant enough to destroy the orthogonal nature of the polarization in the polarized pulse trains.
On the receiver side a polarization controller usually transforms the fluctuating polarizations of the pulse trains into a stable state of polarization (SOP). The polarization controller is often part of control feedback loop in which a portion of the output signal of the polarization controller is tapped off. This portion is transformed by a photodetector into an electrical signal from which a control unit derives a control signal for the polarization controller. Once a stable SOP is recovered, polarization-sensitive optical components, for example a polarization beam splitter, can be used to separate the pulse trains so that an OTDM demultiplexer and photoelectric detectors may reconstruct from the pulse trains the data stream that has been imprinted thereon on the transmitter side.
In ultra-high bit rate OTDM/PDM transmission systems, the successive pulses with alternating polarizations partly overlap. Polarization mode dispersion (PMD) that tends to depolarize the pulses then poses considerable problems, since successive pulses may, when they propagate along the fiber, partly interfere although they had originally orthogonal polarizations. This interchannel interference causes leakage signals (also referred to as noise or simply interference signals) that increases the bit error rate (BER) of the optical transmission system.
From a paper entitled “Polarization Multiplexed 2×20 Gbit/s RZ Transmission using Interference Detection” by S. Hinz et al., currently published in the Internet at http://ont.uni-paderbom.de/publikationen/25372.pdf, it is known to reduce the bit error rate by refining the polarization control at the input side of the receiver. To this end, a sinusoidal frequency modulation is applied to a laser source of the transmitter for generating an interchannel phase modulation. At the receiver side, an interchannel interference signal is detected in the feedback control loop of the polarization controller and then digitally processed so as to obtain a Bessel spectrum thereof. By suitable weighing certain lines of this Bessel spectrum, a control signal for the polarization controller is derived that is independent of the mean interchannel phase difference. The output of the polarization controller is fed to a polarizer and then to an OTDM demultiplexer.
However, the rather complex algorithm that is used in this approach for deriving the control signal from the interference signal requires considerable digital processing power. Further, a frequency modulation has to be applied to the laser source of the transmitter which further increases overall system costs and might impair system performance.
It is, therefore, an object of the present invention to provide a receiver for an optical time-division multiplexed pulse train as mentioned at the outset with reduced system complexity.